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Astronomers discover nearly a thousand hidden galaxies behind the Milky Way

You would think it was pretty hard to hide a galaxy containing tens of billions of stars – but nearly a thousand of them?

Hundreds of new galaxies have been identified within 250 million light years of Earth, hidden behind the glow of our own galaxy, the Milky Way. The discovery is helping us understand the structure of our nearby universe and could challenge some of the assumptions of modern cosmology.

"The region we looked at is a very hard region to study; it's know as the 'Zone of Avoidance', another name for the plane of our own galaxy," Professor Lister Staveley-Smith at the University of Western Australia, told Fairfax Media.

The centre of our galaxy is teeming with dust clouds and billions of stars. Very little optical light is able to penetrate the dense dust and gas. Even when it does, the foreground stars are so densely packed together that they blind our telescopes to the faint glow from more distant objects. Space behind the Milky Way is invisible to us on Earth.

Using novel techniques in radio astronomy, Australian scientist Professor Staveley-Smith and his team were able to detect 883 new galaxies, a third of which have never been seen before and the rest of which were barely smudges.

"The study allows us to build up a picture of where the mass is in the local universe more accurately," Professor Staveley-Smith said.


His team's findings could also help us understand a strange gravitational anomaly known as the Great Attractor, which is dragging us through space at two million kilometres an hour.

"The Milky Way is very beautiful, of course, and it's very interesting to study but it completely blocks out the view of galaxies behind it," said Professor Staveley-Smith, lead author of the study published on Wednesday in the Astronomical Journal.

While intergalactic space is largely empty, at cosmic scales galaxies form intricate structures, known as clusters and superclusters. Understanding how these form helps our understanding of how galaxies are born, live and die – and contributes to our overall cosmological knowledge about the formation and fate of our universe.

Professor Staveley-Smith's study has identified 11 new massive structures among the 883 galaxies. But perhaps it is what it points to beyond this area that is most fascinating.

The Great Attractor is a mysterious region of space that is pulling at our galaxy, and hundreds of thousands of others, with the gravitational force of 10 million billion solar masses. We aren't quite sure what it is.

Until now, most astronomers thought it was a giant cluster, supercluster or an even more exotic object such as a supermassive black hole, about 200 million light years away.

However, this study could point to another conclusion. The findings suggest it is more likely that the Great Attractor is a number of clusters and superclusters in the distance range from about 150 to 600 million light years away.

"In the current cosmological paradigm, it is believed that space tends to be smooth and homogenous above scales of about 300 million light years. Our observations are pushing at that assumption.

"If the upper limit of 600 million light years suggested by our research is the case, then this provides some serious problems for the current model."

Professor Staveley-Smith said: "What we have seen is a very complex region with a lot of mass indeed, a lot of complexity but probably not enough mass to explain the original Great Attractor observations. To explain that, we will need even deeper observations and that will take the SKA [square-kilometre array] telescope to do that."

The SKA project is developing the world's largest radio telescope using sites in South Africa and Australia.

Professor Staveley-Smith is deputy director at CAASTRO, the ARC Centre of Excellence for All-Sky Astrophysics. His team used the 21-centimetre multibeam receiver developed by CSIRO for the Parkes radio telescope.

This receiver is able to observe astronomical phenomena invisible or obscured in visible light by tracking the very narrow frequency range of photons emitted by neutral hydrogen atoms as they flip their spin.

While hydrogen is the most abundant element in the universe, this 21-centimetre wavelength emission is still very faint. This is because, on average, a hydrogen atom only flips its spin every 10 million years.